This invention relates generally to optical couplers and more particularly to optical couplers of multimode fibers to multimode fibers including double clad fibers.
This invention deals with pump modules that provide for the optical coupling or transfer of light from one or more light sources, such as semiconductor lasers, via individual multimode fibers into a multimode fiber or into a multimode cladding, single mode core optical fiber functioning, for example, as a fiber laser or a fiber amplifier. One of the most prevalent applications for such modules is the optical pumping of optical fiber amplifiers (OFA""s), such as 1550 nm Er doped fiber amplifiers (EDFAs) employed in wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) transmission networks.
Such amplifier devices may be comprised of a single mode fiber and, more recently, a double clad fiber, either of which have a core that is doped with an active lasing element, such as a rare earth material. Double clad fibers are disclosed and described in U.S. Pat. Nos. 3,808,549 and 4,815,079. The signal to be amplified is propagated through the core while pump light at an appropriate absorption wave-length is also coupled into the core of the single mode fiber but in the case of a double clad fiber, is coupled into the inner cladding. The double clad fiber is favored in high power systems over the single mode fiber since the inner cladding provides for larger amount of pump power to be supplied into the fiber resulting in higher amplification of the signal output of the fiber amplifier. Thus, one or more diode laser arrays, for example, may be coupled via bulk optics to the input end face of a double clad fiber, i.e., into the double clad fiber inner cladding. Examples of multiple source pumping schemes in conjunction with a double clad fiber are shown in U.S. Pat. Nos. 5,263,036 and 5,268,978.
Of particular interest is to couple the light from multiple semiconductor light sources, via a plurality of multimode fibers, into a double clad fiber. PCT Publication WO 96/20519 (PCT/IT94/00218), published Jul. 4, 1996, discloses the optical coupling of a multimode laser source into a multimode fiber which is fused to a double clad fiber that includes a core for carrying a signal to be amplified. However, PCT Publication WO 96/20519 provides for the coupling of only one multimode laser source multimode fiber to a segment of a double clad fiber.
A recently issued patent to DiGiovanni et al., U.S. Pat. No. 5,864,644, discloses the formation of a fused coupler comprising a plurality of multimode fibers that are fusion-spliced at the combined end to a double clad fiber, which patent is incorporated herein by its reference. As indicated in the patent, the numerical aperture (NA) of the multiple multimode fibers would naturally be less than the numerical aperture (NA) of the double clad fiber. The multimode fibers are tightly bundled together in a close-pack formation and drawn at high temperature to form a fused coupler as shown in the patent, which is a drawing process similar to forming biconical fused couplers but having significantly different properties. As shown in FIG. 1 of the patent, a plurality of such multimode fibers fused to form a tapered bundle and then spliced to a double clad fiber with substantially matching NA""s between the fused bundle and the inner multimode cladding of a double clad fiber.
Fused bundled multimode fibers are known in the art. As set forth in U.S. Pat. No. 4,820,010 to Scifres et al., multimode fibers, illustrated in FIGS. 3 and 4 of the patent, are heated, pulled and fused in a bundle without any accompanying air gaps between the fused fibers as indicated in FIGS. 13 and 14. See also U.S. Pat. No. 4,291,940 that discloses coupler for coupling of two multimode optical fibers via a biconical taper section. Also, U.S. Pat. No. 5,170,458 discloses a coupler for coupling both a multimode fiber and a single mode fiber to a double clad fiber.
U.S. Pat. No. 5,864,644 discloses the bundling together of multimode fibers about a central single mode fiber from which the fused tapered coupler is formed and fusion-spliced with substantially matching NA""s to a double clad fiber and with optical alignment of the single mode fiber core to the double clad fiber core at the point of splice.
While the concepts in these patent references relate to Nxc3x97M fused biconical tapered couplers in the form of optical combiners, they are equally applicable to optical splitters, such as the type illustrated in U.S. Pat. No. 5,355,426.
However, the problem with the type of couplers illustrated in U.S. Pat. No. 5,864,644 is the lack of optical power uniformity or optical power distribution into the coupled multimode or double clad fiber. It is best that the input light from the fused tapered coupled region fill uniformly the entire numerical aperture (NA) of the multimode fiber or double clad fiber splice-coupled to the output of the coupler. This cannot be successfully accomplished by the designs illustrated in U.S. Pat. No. 5,864,644.
Thus, what is needed is a way to couple multiple multimode laser sources, via multiple multimode fibers (MMFs), to a multimode fiber (MMF) or a double clad fiber (DCF) that provides for improved optical power distribution properties in the latter fiber.
According to this invention, an optical coupler, that may function as an optical combiner or an optical splitter, comprises a plurality of combined multimode fibers bundled and fused together and pulled to form a coupler that ha a converging or monotonically decreasing taper down to a minimum diametrical waist followed by a diverging or montonically increasing taper, also referred to as a negative taper extending to a nominal diameter of an output fiber or combined output fibers and is optically coupled thereto such as by means of fusion-splicing. The minimum diametrical waist is sufficiently small so that the input light from the combined multimode fibers substantially fills the entire numerical aperture (NA) of the output fiber so that, upon divergence from the waist, the NA of the output fiber will be substantially filled with propagating light distributed uniformly within the waist. The waist is not so small as to bring about substantial loss of the propagating light. The output fiber may be a multimode fiber or a plurality of combined and fused multimode fibers having an NA input substantially matching the NA output from the diverging taper of the coupler. The output fiber may also be a core-doped double clad fiber where the NA output substantially matching the NA output from the diverging taper is the double clad fiber inner cladding, also referred to at times as the pump cladding. The diverging taper may also be a portion of the input of the output fiber. In another embodiment, a single mode fiber may be combined with a plurality of multimode fibers to form a fused converging-to-diverging taper coupler having an output substantially matching the NA of the NA of the output fiber. Disclosed are several ways for forming the minimum diametrical waist followed by the monotonically increasing taper.
In a disclosed embodiment, a multimode fibers/single mode fiber to double clad fiber optical (MMFs/SMF/DCF) coupler comprises a single mode fiber with a first core, a plurality of multimode fibers bundled around said single mode fiber forming a bundled arrangement and a double clad fiber having an inner cladding with a second core. The bundled arrangement is fused and, then, an end of the double clad fiber is butt coupled to an end of the fused, bundled arrangement with the first and second cores in aligned relation. The fused, bundled arrangement has a diametrical cross-section substantially equal to the diametrical cross-section of the double clad fiber inner cladding. The multimode fibers may be pre-tapered to provide, in combination, the diametrical cross-section substantially equal to the diametrical cross-section of the double clad fiber inner cladding. Alternatively, the optical coupler may be provided to have a tapered shape and is cut at a length to provide the diametrical cross-section substantially equal to the diametrical cross-section of the double clad fiber inner cladding. Also, alternatively, a portion of the single mode fiber cladding surrounding the first core may be removed prior to forming the bundled arrangement.
The coupler of this invention may be coupled to a multimode fiber for optical power delivery to an optical component or coupled to double clad fiber functioning as a fiber laser or fiber amplifier.
In another embodiment of this invention, an optical coupler comprises a multimode fiber having a multimode core and a double clad fiber having an inner cladding with a single mode core. Selected segments of the fibers are twisted and fused together to form an optical coupling between the multimode fiber and the inner cladding of the double clad fiber. Preferably, a portion the cladding of the multimode fiber is removed or tapered so that the coupling efficiency between the multimode fiber and the double clad fiber is enhanced.
A further embodiment relates to a method of forming an optical coupler comprising the steps of providing a plurality of multimode fibers, tapering the end portions of the multimode fibers, bundling the tapered end portions of the multimode fibers about a single mode fiber, and fusing all the bundled fibers together. A double clad fiber having an inner cladding with a predetermined diameter is provided for butt coupling an end of the fused fibers to an end of the double clad fiber. The method further includes the step of selecting the amount taper of the multimode fiber end portions so that the resulting diameter of the bundled tapered end portions together with the single mode fiber approximates the predetermined diameter after their fusing.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.